Iranian riot police have clashed with hundreds of pro-reform protesters in central Tehran, according to one witness, who claims that dozens of people have been detained.

"There are hundreds of riot police and plainclothes, beating people who gathered to support (opposition leader Mirhossein) Mousavi," the witness said.

"There are hundreds of protesters at Haft-e Tir square."

Tweets posted on Twitter, which cannot be verified, suggest that around 10,000 people are involved in the demonstrations in four main areas of central Tehran: "Vali Asr; 7 Tir; Coledge Sq; Baharestan".

One tweet says: "A young lad with a phone nr Cinema was grabbed by security. Crowd freed him while fighting off teargas attacks.

"As the crowd gathered near the cinema, guards showed force by beating an elderly person, crowd came to rescue."

In the latest unrest over last month's disputed election, the witness said demonstrators were chanting slogans against President Mahmoud Ahmadinejad and the government, including: "Ahmadinejad - resign, resign" and "Death to dictators".

"Riot police are taking dozens of protesters into their cars and they are taking them away," the witness said.

Such demonstrations were banned following the June 12 election, which the opposition alleges was rigged in favour of Ahmadinejad.

The latest protests came after clashes between police and protesters for the first time in weeks on Friday, after former President Akbar Hashemi Rafsanjani declared the Islamic Republic in crisis and said there were doubts about the election result.

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